1. Field of Invention
Localized heating of glass-enamel frits with infrared and visible/infrared radiation is used to form glass-based seals between substrates, particularly for glass substrates.
2. Description of Related Art
The standard method for the formation of glass-based seals is thermal heating. Thermal sealing is generally a slow process, typically involving heating cycles on the order of 100 minutes, and time at sealing temperature on the order of 20-60 minutes. The cool down part of the cycle is generally slow enough to allow a significant degree of annealing in order to minimize any stress build up in the seals and at the interfaces. In addition, since the entire assembly is heated in this process, it requires thermally stable components within the seal and requires considerable energy consumption.
Some alternate approaches to thermal sealing can be grouped under the concept of localized heating. Localized heating implies that the sealing energy is primarily supplied to or primarily absorbed by the seal material and sealing area, and does not significantly heat the rest of the device, substrates, etc. The energy can be supplied as any kind of electromagnetic radiation, from high energy X-rays to long wavelength radio waves. In the case where the energy is supplied as a small spot or line, the beam can be aimed strictly at the seal area. Therefore, one of the important advantages of localized heating is the protection of any sealed or attached components from radiation and thermal damage. The protected component could be a thin-layer solar cell, a tempered glass substrate, a MEMS device, or an OLED device for example. Second, as previous implied, there should be a considerable energy savings since the entire assembly is not heated to sealing temperature.
Unfortunately, localized heating approaches come with numerous obstacles to overcome in order to be practical for commercial purposes. The most significant differences and difficulties compared to thermal sealing are adequate control of the sealing temperature, large ranges of sealing temperatures reached in different sealing areas, very large thermal gradients that commonly cause thermal shock fractures in the seals and/or substrates, and the very short sealing times that limit the amount of glass flow for forming a strong seal.
Much research has been done on the laser sealing of glass-based seals. This is another localized heating approach, and it encounters many of the same obstacles as other localized heating approaches. With spot-to-spot exposure of high energy packets for very short times, on the order of 100 milliseconds (0.1 sec), extremely large thermal gradients are encountered and there is typically inadequate time to obtain enough glass flow to seal all the gaps in the seal area.
A problem of focused IR sealing with glass-based enamels is reaching the temperatures necessary for melting some of the glass powders. In general, the energy density from a focused IR lamp is less than 1/10 that of a laser spot. Since the maximum temperature will depend on the equilibrium of energy input from the IR lamp and energy loss due to heat flow, lower temperatures are observed in focused IR sealing. Applicants have solved this problem in several ways. First, we have increased the energy input rate by using higher energy-density lamps, adapting the geometry of the sealing material, increasing the absorption coefficient of the seal material, and modifying the direction of the beam. Second, we have learned to control the energy output rate from heat flow by applying layers of various insulation values between the substrates and heat sinks. Third, we have developed and chosen glasses with a variety of flow temperatures.
One of the common problems encountered with many fast-sealing techniques is the formation of voids as well as foaming in the seal material. This problem is primarily caused by pre-existing voids and by binder burnout in either the as-deposited paste or in the dried paste, when the binder's combustion products and any remaining volatiles have inadequate time to be eliminated from the enamel during the sealing fire. The inventors have found that the use of pre-fired enamels can be used in some cases to significantly help eliminate voids and foaming. Since in most cases the enamel can be applied to and fired on the blank substrate, the components on the active substrate would not be affected by the pre-fire. Light sanding of the pre-fired enamel on the blank substrate can also be done to enhance its activity for bonding to the active substrate.
Accordingly improvements are sought in sealing processes.